1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiple Access (OFDMA) mobile communication system, and in particular, to a method and apparatus for positioning pilots in an OFDMA mobile communication system.
2. Description of the Related Art
Recently, in mobile communication systems, intensive research and development is being conducted on Orthogonal Frequency Division Multiplexing (OFDM) as a useful scheme for high-speed data transmission over wire/wireless channels. OFDM, a scheme for transmitting data using multiple carriers, is a type of Multi-Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbol streams and modulates each of the parallel symbol streams with multiple orthogonal sub-carriers, or multiple sub-carrier channels, before transmission.
The system employing MCM was first applied to military communications in the late 1950s, and OFDM that overlaps multiple orthogonal sub-carriers, even though it has developed from the 1970s, has had a limitation in its application to the actual system because of the implementation difficulty of orthogonal modulation between multiple carriers. However, since Weinstein et al. submitted in 1971 that OFDM-based modulation/demodulation enables efficient processing using Discrete Fourier Transform (DFT), technology development for OFDM has been rapidly carried out. In addition, as OFDM uses a guard interval and a scheme of inserting a Cyclic Prefix (CP) symbol in the guard interval is known, the negative influences on the system for multiple paths and delay spread have remarkably been reduced.
Owing to the technical development, the OFDM technology is being broadly applied to digital transmission technologies such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), Wireless Asynchronous Transfer Mode (WATM), etc. However, OFDM could not be broadly used due to its hardware complexity, but its realization is now possible with the recent development of various digital signal processing technologies including Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT). OFDM, though it is similar to the conventional Frequency Division Multiplexing (FDM), maintains orthogonality between multiple sub-carriers during transmission, thereby obtaining optimal transmission efficiency during high-speed data transmission. OFDM, as it has high frequency utilization efficiency and is robust against multi-path fading, can obtain optimal transmission efficiency during high-speed data transmission. In addition, OFDM, because it overlaps the frequency spectra, has high frequency utilization efficiency and is robust against frequency selective fading and multi-path fading. Further, OFDM can reduce an Inter-Symbol Interference (ISI) effect using the guard interval, and can simply be utilized in the design of an equalizer. Moreover, OFDM, as it is robust against impulse noise, tends to be used for communication systems.
In wireless communication systems, the high-speed, high-quality data services are generally impeded by the channel environment. The channel environment frequently varies not only due to Additive White Gaussian Noise (AWGN) but also due to power variation of a received signal, caused by fading, shadowing, a Doppler effect based on movement and frequent velocity change of a terminal, interference by other users and multi-path signals, etc. Therefore, there is a need to efficiently overcome the impediment factors in order to support the high-speed, high-quality data services in wireless communications.
FIG. 1 is a diagram illustrating pilot positioning in a conventional block having 8 OFDM symbols in a time domain and 15 tones in a frequency domain.
In OFDM, a modulation signal is located in 2-dimensional resources composed of time and frequency. Resources on the time axis are divided into different OFDM symbols, and resources on the frequency axis are divided into different sub-carriers, which are orthogonal to each other.
As shown in FIG. 1, data symbols are transmitted over uncolored (or white) tones, and pilot symbols are transmitted over colored (or black) tones. In addition, the pilot symbols are transmitted at intervals of 7 frequency tones (d=7) in the frequency domain. When the pilot symbols are transmitted at regular intervals, a data symbol demodulation performance after channel estimation at a receiver, is increased, as compared to when the pilot symbols are transmitted at irregular intervals. For example, in FIG. 1, pilots are positioned in 1st, 8th and 15th sub-carriers on the frequency axis. To receive data positioned in 2nd to 7th sub-carriers, the receiver estimates a channel through interpolation using the 1st and 8th pilot symbols, and demodulates the data. Similarly, to receive data positioned in 9th to 14th sub-carriers, the receiver estimates a channel through interpolation using the 8th and 15th pilot symbols, and demodulates the data. Unlike FIG. 1, if pilots are positioned in 1st, 4th and 15th sub-carriers on the frequency axis, demodulation performance for 2nd and 3rd data symbols may increase with the use of channel estimation on 1st and 4th pilot symbols, but data symbols positioned between 4th and 15th pilot symbols may suffer from a decrease in the channel estimation performance due to the long pilot interval, causing a possible decrease in the entire data demodulation performance. That is, if the pilots are transmitted at irregular intervals, the channel estimation performance decreases for the data symbols located between the pilots transmitted at the longer interval, causing a reduction in a data reception rate. Therefore, a pilot positioning scheme is used in which an interval between pilot symbols is regular in the frequency domain.
FIG. 2 is a diagram illustrating pilot positioning in a conventional block having 8 OFDM symbols in a time domain and 16 tones in a frequency domain, and FIG. 3 is a diagram illustrating pilot positioning in a conventional block having 8 OFDM symbols in a time domain and 8 tones in a frequency domain.
Shown in FIG. 2 is a block having 8 OFDM symbols in the time domain and 16 tones in the frequency domain. As pilot tones for channel estimation are selected at intervals of 7 pilot tones in the frequency domain, an interval (or distance) d between the pilots is 7, so the interval between the pilot tones in the frequency domain is constant. However, for the time domain, the remaining tones except for 2 tones located in the center are all used.
This block becomes a hopping unit for resource assignment in the OFDMA mobile communication system, and the system transmits data and pilots while hopping the entire frequency band in units of blocks over the passage of time.
A size of the block used in FIG. 3 is ½ that of the size of the block shown in FIG. 2 on the frequency axis, and an interval between pilot symbols in the frequency domain is d=3. The format of FIG. 3 can be used only when an FFT size is less than 512, and because its d value is relatively small, this block is effective for large delay spread.
In FIGS. 2 and 3, because the interval between pilot symbols in the frequency domain is regular as in the conventional technology, the bottom tones of the block are not used for transmitting pilot symbols. That is, data symbols are transmitted over the bottom frequency tones of the block. Therefore, for channel estimation for the bottom frequency tones, extrapolation should be performed using the pilot symbols located in the just upper level of the bottom. Compared to the interpolation that uses pilot symbols located in both ends of the data symbols, the extrapolation may cause a reduction in the channel estimation performance.